86 research outputs found
Modeling the near-infrared lines of O-type stars
We use a grid of 30 line-blanketed unified stellar photosphere and wind
models for O-type stars; computed with the code CMFGEN in order to evaluate its
potential in the near-infrared spectral domain. The grid includes dwarfs,
giants and supergiants. We analyse the equivalent width behaviour of the 20
strongest lines of hydrogen and helium in spectral windows that can be observed
using ground-based instrumentation and compare the results with observations.
Our main findings are that: i) HeI/HeII line ratios in the J, H and K bands
correlate well with the optical ratio employed in spectral classification, and
can therefore be used to determine the spectral type; ii) in supergiant stars
the transition from the stellar photosphere to the wind follows a shallower
density gradient than the standard approach followed in our models, which can
be mimicked by adopting a lower gravity in our prescription of the density
stratification. iii) the Brackett gamma line poses a number of peculiar
problems which partly might be related to wind clumping, and iv) the Brackett
alpha line is an excellent mass-loss indicator. For the first and last item we
provide quantitative calibrations.Comment: 14 pages, 7 figures, accepted by A&
An infrared view of (candidate accretion) disks around massive young stars
Near-infrared surveys of high-mass star-forming regions start to shed light onto their stellar content. A particular class of objects found in these regions, the so-called massive Young Stellar Objects (YSOs) are surrounded by dense circumstellar material. Several near- and mid-infrared diagnostic tools are used to infer the physical characteristics and geometry of this circumstellar matter. Near-infrared hydrogen emission lines provide evidence for a disk-wind. The profiles of the first overtone of the CO band-heads, originating in the inner 10 AU from the central star, are well fitted assuming a Keplerian rotating disk. The mid-infrared spectral energy distribution requires the presence of a more extended envelope containing dust at a temperature of about 200 K. CRIRES observations of CO fundamental absorption lines confirm the presence of a cold envelope. We discuss the evolutionary status of these objects
Identification of the ionizing source of NGC 2024
We propose the late-O, early-B star IRS2b as the ionizing source of the Flame
Nebula (NGC 2024). It has been clear that such a hot, massive star must be
present in this heavily obscured region, and now it has been identified. New
near-infrared photometry shows that IRS2b is the most luminous and hottest star
in the young star cluster embedded in the center of NGC 2024. The near-infrared
observations (5' x 5') cover ~90 % of the HII region detected in radio
continuum radiation, making the probability very low that the ionizing star is
not present in the field. A K-band spectrum of IRS2b obtained with ISAAC on the
Very Large Telescope indicates that the spectral type of IRS2b is in the range
O8V - B2V. Additional arguments indicate that its spectral type is likely
closer to O8 than to B2. The corresponding amount of ionizing radiation is
consistent with published radio continuum and recombination line observations.Comment: 7 pages, 4 figures. Figure 1 included as jpeg. Accepted for
publication in Astronomy & Astrophysic
On the central ionizing star of G23.96+0.15 and near-IR spectral classification of O stars
Aims: A near-infrared study of the main ionizing star of the ultracompact HII
region G23.96+0.15 (IRAS 18317-0757) is presented, along with a re-evaluation
of the distance to this source, and a re-assessment of H- and K-band
classification diagnostics for O dwarfs; Methods: We have obtained near-IR
VLT/ISAAC imaging and spectroscopy of G23.96+0.15, plus archival imaging from
UKIRT/UFTI. A spectroscopic analysis was carried out using a non-LTE model
atmosphere code; Results: A quantitative H- and K-band classification scheme
for O dwarfs is provided, from which we establish an O7.5V spectral subtype for
the central star of G23.96+0.15. We estimate an effective temperature of Teff ~
38 kK from a spectral analysis; Conclusions: A spectroscopic distance of 2.5
kpc is obtained for G23.96+0.15, substantially lower than the kinematic
distance of 4.7 kpc, in common with recent studies of other Milky way HII
regions. Such discrepancies would be alleviated if sources are unresolved
binaries or clusters.Comment: 5 pages, 3 figures, accepted for Astronomy & Astrophysic
The peculiar circumstellar environment of NGC2024-IRS2
We re-examine the nature of NGC2024-IRS2 in light of the recent discovery of
the late O-type star, IRS2b, located 5 arcsec from IRS2. Using L-band
spectroscopy, we set a lower limit of Av = 27.0 mag on the visual extinction
towards IRS2. Arguments based on the nature of the circumstellar material,
favor an Av of 31.5 mag. IRS2 is associated with the UCHII region
G206.543-16.347 and the infrared source IRAS 05393-0156. We show that much of
the mid-infrared emission towards IRS2, as well as the far infrared emission
peaking at ~ 100 micron, do not originate in the direct surroundings of IRS2,
but instead from an extended molecular cloud. Using new K-, L- and L'-band
spectroscopy and a comprehensive set of infrared and radio continuum
measurements from the literature, we apply diagnostics based on the radio
slope, the strength of the infrared hydrogen recombination lines, and the
presence of CO band-heads to constrain the nature and spatial distribution of
the circumstellar material of IRS2. Using simple gaseous and/or dust models of
prescribed geometry, we find strong indications that the infrared flux
originating in the circumstellar material of IRS2 is dominated by emission from
a dense gaseous disk with a radius of about 0.6 AU. At radio wavelengths the
flux density distribution is best described by a stellar wind recombining at a
radius of about 100 AU. Although NGC2024/IRS2 shares many similarities with
BN-like objects, we do not find evidence for the presence of a dust shell
surrounding this object. Therefore, IRS2 is likely more evolved.Comment: 15 pages, 10 figures. Accepted for publication in Astronomy &
Astrophysic
- …